Editors' ChoiceNeuroscience

Sunshine for your mind

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Sci. Signal.  03 Jul 2018:
Vol. 11, Issue 537, eaau6107
DOI: 10.1126/scisignal.aau6107

Moderate exposure to ultraviolet light stimulates glutamate synthesis in the brain and improves learning and memory.

Although high doses of ultraviolet (UV) radiation promote skin inflammation, damage, and cancers, low doses are used clinically to treat some skin diseases because they reduce inflammation. Exposure to moderate amounts of sunlight is beneficial not only because it stimulates the biosynthesis of vitamin D but also because it enhances mood, cognition, and learning (see Chantranupong and Sabatini). Zhu et al. found that moderate exposure of shaved mice to UVB radiation increased the abundance of urocanic acid (UCA) in the skin, blood, and brain. UCA is an intermediate in the conversion of histidine to glutamate in peripheral tissues. In addition to UCA, the amounts of histidine, glutamate, and two other intermediates in the conversion of histidine to glutamate also increased in the brain after UV exposure. The biosynthesis of glutamate from histidine was thought only to occur in peripheral tissues, but enzymes that mediate this conversion were present in neurons from various brain regions, and neurons from brain slices synthesized glutamate from histidine in vitro. (Although glutamate is an important excitatory neurotransmitter, neurons of the central nervous system typically acquire glutamate by active transport across the blood-brain barrier or synthesize it from glutamine.) UV exposure stimulated glutamine biosynthesis and glutamatergic synaptic transmission in the brain and improved performance in motor learning and recognition memory tests. Thus, in addition to boosting mood by stimulating the release of endorphins, sunlight also stimulates learning and memory by enhancing glutamate signaling in the brain.

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